1. Field of the Invention
This invention relates to the production of artificial graphite and more particularly to a new and improved furnace for graphitizing carbon bodies to artificial graphite.
2. Description of the Prior Art
Artificial carbon is made by heating amorphous carbon at high temperature converting the carbon to graphite which is a crystalline form of carbon. The source of carbon may be coal, but is preferably derived from petroleum in the form of low-ash coke. The coke is mixed with coking binder such as coal pitch and formed into the required shape called "green" body or green electrode. After preliminary heating to a temperature up to about 2000.degree. C, the carbon body is heated above about 2200.degree. C to a temperature of nearly 3000.degree. C and retained at the high temperature for a sufficient time for the formation of the hexagonal crystal pattern. The thermal conductivity of graphite is higher than carbon and this combined with a low coefficient of thermal expansion gives it high resistance to shock. Although carbon and graphite are extensively used in electrical applications, graphite, despite its high cost, is usually selected as anodes for electrical applications because of higher purity, higher electrical conductivity, greater ease of machining and high oxidation resistance.
Discontinuously operated furnaces for graphitization of carbon bodies are known in the art as illustrated by the Acheson furnace and consist basically of two graphite electrodes which are disposed in the end walls of the furnace, between which electrodes the bodies to be graphitized are layered between granular resistance and isolation material. In order to heat the carbon bodies, the graphite electrodes which are short-circuited by the furnace content, are electrically connected to a current source, whereby the generated Joule's heat causes a rise of the furnace temperature to 2200.degree. up to approximately 3000.degree. C. Depending on the size of the furnace, the period of time required to heat up the furnace is approximately 1 - 3 days and the following cooling period is approximately 5 - 12 days.
A considerable disadvantage of the Acheson process is the unfavorable ratio of the heating and cooling periods and the resulting low production efficiency. The large quantities of resistance and isolation materials used in the process require special transport, sorting and cleaning devices. The carbon bodies must be layered with great care to avoid resistance variations, without fully eliminating localized resistance variations and the resulting temperature peaks during the heating up period which deteriorate the quality of the graphite bodies. Finally, the efficiency of the Acheson method is comparatively low because in addition to the graphite bodies, a large quantity of resistance material is also heated up to the graphitization temperature and only a small part of the thermal energy supplied is recovered. Further disadvantages of this method are the difficulties to contain and discharge the poisonous gases which are generated during graphitization, for example, sulfur dioxide and carbon monoxide. Also resistor material becomes attached to the graphite bodies and the former has to be removed by grinding or other mechanized operations.
It is known to clamp one or several carbon bodies which are arranged in a row between two graphite electrodes and to heat the body to graphitization temperature by the direct passage of electric current. In this procedure at least one electrode is movable and is pressed by a force which acts in the direction of the furnace, against the carbon bodies which butt against the second electrode, in order to avoid undesirable high voltages at the contacts or even a discontinuity of the current path. To avoid oxidation of the carbon bodies, the whole furnace space is filled with granular isolation materials, for example, coke. While this type of furnace permits faster heating up time, the cooling period is only insignificantly shortened and the adhesion of the isolation material to the graphite bodies and the turnover of large quantities of isolation material is unchanged. Furthermore, it is impossible to completely contain the noxious gases that emerge from the open furnace by exhaust hoods which could be mounted above the furnace.